The thermochromic property of cholesteric liquid crystal compounds has invited considerable effort for their application to thermometer inventions. Many of the liquid crystal thermometers described in prior art are alleged for measurement of human body temperature: however, none has been a commercial success because they all fail in one or more of their attributes to satisfy the needs of the medical community.
The liquid crystal forehead thermometer is a good example of one such product. Because of its low resolution, +1.degree. C., it has principally served as a screening device for fever, requiring subsequent confirmation of true body temperature with a mercury-in-glass or electronic clinical thermometer. This is not, however, its only serious drawback. The forehead has been found to be a unreliable site for representing core temperature, failing more than 30% of the time to detect fever (false negatives).
A medically acceptable clinical thermometer based on liquid crystal color changes must meet the same exacting standards for range (35.degree.-41.degree. C.), resolution (0.1.degree. C.), accuracy (0.1.degree. C. in the critical range, 0.2.degree. C. elsewhere), and stability as do the mercury in glass and electronic clinical thermometers. Furthermore, it must be (i) designed to measure the temperature at a site which is accepted in the medical community is faithfully representing core temperature; e.g., rectum, sublingual cavity or axilla, (ii) easy to read, clean and reset between uses, (iii) safe and comfortable when used at the site, (iv) and retain its accuracy for at least five years during storage and distribution when subjected to temperature extremes of +20.degree. to 60.degree. C. it should also exhibit some special features which result in benefits not shared by competitive products; e.g., less expensive, easier to use, unbreakable, no power requirement, child friendly, etc.
U.S. Pat. No. 3,974,317, incorporated herein by reference, describes a thermometric composition which fulfills the requirements regarding range, resolution, accuracy and stability. This patent describes a cholesteric liquid crystal system which can be used to construct thermometric elements capable of recording numerous increments in temperature from a single basic composition in a facile and economic manner.
Each thermometric element of this invention comprises a plurality of separate compositions having identical colors when viewed on an inert black background. Each distinct composition is capable of recording temperature by a visual change in color and is comprised of a mixture containing:
a. a first cholesteric liquid crystal system characterized by exhibiting color in the cholesteric state at a first temperature and changing from this state to a second state exhibiting a different color at a second temperature, and PA1 b. a second component other than (a) which is a chemically inert substance miscible with (a); the same second component in differing percentages by weight thereof, being utilized in each composition wherein said liquid crystal systems are identical, the differing amounts of said second component in each composition wherein the liquid crystal systems are identical being in a predetermined weight range wherein there is a predictable variability in a curve in which the temperature at which said visual change in color takes place is plotted against percent by weight of said second component.
This '317 patent thus demonstrates that it is possible to change the phase transition temperature, referred to in the art as the clearing point of a liquid crystal system in a predictable manner by varying the amount of the second component (b) in the composition.
An illustrative example of a composition taught in the 'patent is a liquid crystal system comprising 57.9% cholesteryl oleyl carbonate 30.7% cholesteryl chloride and 11.4% cholesteryl-n butoxphenyl carbonate as component (a). This composition has a clearing point at 54.0.degree. C. When mineral oil, component (b), is added to (a), the clearing point temperature is depressed as a linear function of the percent by weight of the mineral oil added to (a). A plot of the clearing point temperature vs. weight percent mineral oil has a negative slope of 2.98.degree. C. per one percent change in the mineral oil content of the composition, e.g., a composition containing 5.70 weight percent mineral oil would exhibit a clearing point at 37.0.degree. C., whereas a composition containing 5.67% weight mineral oil would exhibit a clearing point 0.1.degree. higher or 37.1.degree. C.
Because the colors of all of the compositions containing both (a) and (b) are identified both below and above their respective clearing points, and because the change in color is not subjective, temperature differences as small as 0.1.degree. C. can be easily resolved, thus making the compositions of U.S. Pat. No. 3,974,317 ideally suited for application in a clinical thermometer.
This chemistry, while necessary, is insufficient for making a clinical thermometer. What also must be specified are the neat sealable substrate and transparent covering film that contain the liquid crystal compositions and are inert relative to these compositions. Constraints on these materials are set forth in U.S. Pat. No. 4,064,872, incorporated herein by reference. Here it is taught that for the preparation of the thermometers useful for medical diagnosis, the separate films comprising the heat sealable sheet material and the carrier substrate should contain less than 1 mg per square meter of components which will react with the liquid crystals, either during manufacture or storage. These potentially reactive materials may be residuals from the manufacturing process of the film such as monomers, solvents, inhibitors or processing aids which may react with or dissolve in the liquid crystal composition selected, resulting in modification of the temperature at which color change will take place. The '872 patent teaches the use of polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) coated laminates as the heat sealing material. This patent emphasizes that the materials selected to enclose the liquid crystal compositions should be of as low thermal mass as is possible consistent with sufficient durability to allow for repeated use.
The '872 patent additionally describes a method for constructing a clinical thermometer from these compositions by arranging them in a dot matrix array. Combining the teachings of this patent with those described in the '317 patent results in a clinical thermometer with the requisite precision, stability and accuracy set forth in the above discussion. Furthermore, this chemistry has been found to be nontoxic when tested on laboratory animals at doses where comparable levels of a common toothpaste led to fatalities in all the animals tested. Plastic thermometers made with this chemistry are unbreakable and can be used hundreds of times without loss of efficacy.
Thermometers made using the combined teachings of '317 and '872 patents suffer from a serious technical shortcoming relating to readability. Because of the small size of the individual dots, 1 mm diameter, the low contrast between the green liquid state and the gray focal conic state, and the relatively short duration of the signal, 15-20 seconds, before reversion begins, those unfamiliar with reading the thermometer or those attempting to read it in lighting of low intensity will experience difficulty. Because of this deficiency, clinical thermometers made using these teachings have experienced limited commercial success.
When most liquid crystal thermometers are removed from one environment in their range of transition to a lower temperature, the signal fades so rapidly that it is not possible to obtain an accurate temperature determination of the first environment. This is for two reasons. Firstly, like all thermometers, those made of liquid crystals are of low thermal mass and cool quickly. Secondly, nearly all liquid crystal compositions respond with time constants of less than one second, whether it is exposed to a temperature increase or decrease and thus display exceedingly short memory.
The chemistry taught in '317 does not differ from other liquid crystal thermometer chemistries in this respect for its transition from its clear isotropic state to a cloudy focal conic state is essentially instantaneous. For reasons which are not well understood, however, the particular combination of liquid crystals described in the '317 patent becomes trapped briefly in this focal conic state undergoing a rather sluggish transition from this state to the brightly colored liquid crystal state. This brief memory is quite sensitive to the temperature difference between the test and reading environments. For example, a liquid crystal thermometer removed from a 37.degree. C. mouth and returned to a room temperature of about 20.degree. C. would retain a high contrast signal for approximately 20 seconds before disappearing; whereas, the same thermometer withdrawn from a mouth at 40.degree. C. and returned to room temperature would retain its signal for only 10 seconds.